228 Walter Stiles and Ingvar Jorgensen. 
By means of conductivity measurements of the expressed sap, 
Bouyoucos (9) has been able to show that the quantity of salts 
present in the sap ot wheat seedlings grown in water-culture depends 
on the concentration of the solution in which the plants were grown. 
The plant juices were obtained by pressing the seedlings in a mortar 
until only white fibre was left. The juice was then diluted with 
distilled water to a definite volume and its conductivity measured. 
Recently Dixon and Atkins (10, 11, 12) in a series of investiga¬ 
tions on osmotic pressure in plants, have shown that the electrical 
conductivity of expressed sap increases with increased pressure, 
thus confirming the result of Mameli mentioned above. They 
explain this as due to the semi-permeability of the protoplasmic 
membrane, which allows water to pass out readily, but not the 
dissolved salts. But with even weak pressure some cells will burst 
and then sap accordingly passes out and mixes with the water 
extruded from the compressed but intact cells. With increasing 
pressure however more cells are broken, and so as the pressure 
used increases, there increases the concentration of the expressed 
liquid as regards dissolved substances, and consequently also its 
electrical conductivity. 
So in order to get an extract which shall really represent the 
sap of the cells it is necessary to render the cell membranes 
completely permeable before pressure is applied. Chloroform and 
toluene vapour were made use of for this purpose, but it was found 
that the most satisfactory method consisted in freezing the tissues 
in liquid air before pressing out the juice. Much higher numbers 
were obtained for osmotic pressures and electrical conductivity by 
this method than had been obtained previously by the same authors. 
The results of measurements of the conductivity of numerous saps 
extracted in this way are given by Dixon and Atkins. These results 
on the whole confirm those obtained by the more inexact methods 
employed in earlier researches. Thus it was found that the osmotic 
pressure and electrical conductivity of the sap increases in passing 
upwards from the root to the shoot. The difference in the 
conductivity of various saps is considerable, the extremes being a 
specific conductivity of 1141 x 10 5 for the bases of etiolated leaves 
of Apium graveolens, and 112 x 10-* for the fruit of Vitis vinifera. 
The latter however had the highest osmotic pressure of all the saps 
investigated. In fact the low conductivities of organic acids and 
salts which are abundant in fruits explain the low numbers obtained 
for the conductivities of fruit saps. 
